Well it looks like it could be a cool proj. Here are some issues that I've noticed off the bat: How are you going to run the drive chains? How are you getting fuel to the 2nd engine? How will you synch the throttles? You will also probably need to reinforce the frame. Get some tubing and weld a backbone from the steering tube to the 1st seat post and then from there to the 2nd. The trick would be to get both engines to fire. Don't forget that the clutches will have to be synched somehow also, because I assume that the person on the front seat would be controlling all that, unless you and your passenger are really REALLY good. Another idea, might be to simply get a 110cc pocket bike engine, mount it and just use that. They can come automatic and with both engines like you have it, you essentially have at least a 100cc bike anyway. You'd still need to reinforce the frame and then get creative with some jack shafts probably. Just my two cents.
